Windows did some updates last night and rebooted my PC :(
How can I, if at all, recover and un-saved notepad document that was sitting open?
Notepad doesn't auto-save its open file so unfortunately you have lost your work.
As pointed out by ChrisF, your data is lost because of the reboot. Sorry.
To prevent such problems in the future, I'd recommend disabling the automatic reboot. Microsoft claims this is "by design", but do provide a way to disable it, by editing the registry:
Of course you should still reboot as soon as possible to avoid problems (with unpatched security holes, and with problems because of outstanding changes), but you can now do so at your leisure.
This should only be an issue for Windows XP SP1 and below; apparently in XP SP2 and later versions, Windows will no longer reboot on its own. It will still pop up reminders prompting you to restart, but it will only restart if you click the button in the popup.